Large Binary Files
If you have a large amount of binary data, or perhaps just a large file, that you want to sync between devices then instead of inserting this into a document as bytes you should make use of the attachments feature.
Attachments do not get synced between devices by default, even if they are part of a document that is being synced between devices. This is because they could be very large files that a given device doesn't need. Instead an attachment must be explicitly fetched using an attachment token that will be present in the document that the attachment is linked to.
For more information, see the Platform Manual > Attachment.